Sources of moisture

Typical sources of moisture are large bodies of water such as the Atlantic and Pacific oceans as well as the Gulf of Mexico.

Water temperature also plays a large role in how much moisture is in the atmosphere. Recall from the Ocean Section that warm ocean currents occur along east coasts of continents with cool ocean currents occur along west coasts. Evaporation is higher in warm ocean currents and therefore put more moisture into the atmosphere than with cold ocean currents at the same latitude.

Therefore, in the southeastern U.S. the warm water from the two moisture sources (Atlantic Ocean and Gulf of Mexico) helps explain why there is much more rain in that region as compared to the same latitude in Southern California.

Instability

Air is considered unstable if it continues to rise when given a nudge upward (or continues to sink if given a nudge downward). An unstable air mass is characterized by warm moist air near the surface and cold dry air aloft.

In these situations, if a bubble or parcel of air is forced upward it will continue to rise on its own. As this parcel rises it cools and some of the water vapor will condense forming the familiar tall cumulonimbus cloud that is the thunderstorm.

Sources of Lift (upward)

Typically, for a thunderstorm to develop, there needs to be a mechanism which initiates the upward motion, something that will give the air a nudge upward. This upward nudge is a direct result of air density.

Some of the sun's heating of the earth's surface is transferred to the air which, in turn, creates different air densities. The propensity for air to rise increases with decreasing density. This is difference in air density is the main source for lift and is accomplished by several methods.

Differential Heating

The sun's heating of the earth's surface is not uniform. For example, a grassy field will heat at a slower rate than a paved street. A body of water will heat slower than the nearby landmass.

This will create two adjacent areas where the air is of different densities. The cooler air sinks, pulled toward the surface by gravity, forcing up the warmer, less dense air, creating thermals.

Fronts, Drylines and Outflow Boundaries

Fronts are the boundary between two air masses of different temperatures and therefore different air densities. The colder, more dense air behind the front lift warmer, less dense air abruptly. If the air is moist thunderstorms will often form along the cold front.

Drylines are the boundary between two air masses of different moisture content and separates warm, moist air from hot, dry air. Moist air is less dense then dry air. Drylines therefore, act similar to fronts in that a boundary exists between the two air masses of different densities.

The air temperature behind a dryline is often much higher due to the lack of moisture. That alone will make the air less dense but the moist air ahead of the dryline has an even lower density making it more buoyant. The end result is air lifted along the dryline forming thunderstorms. This is common over the plains in the spring and early summer.

Outflow boundaries are a result of the rush of cold air as a thunderstorm moves overhead. The rain-cooled, more dense, air acts as a "mini cold front", called an outflow boundary. Like fronts, this boundary lifts warm moist air and can cause new thunderstorms to form.

Terrain

As air encounters a mountain it is forced up because of the terrain. Upslope thunderstorms are common in the Rocky Mountain west during the summer.